Sensors for measuring pressure within a body cavity, organ or vessel have been in use for many years. For example, when a patient's heart does not function normally due to a genetic or acquired condition, various treatments may be prescribed to correct or compensate for the condition. Pharmaceutical therapy may be prescribed for the patient or a pacemaker may be implanted in the patient to improve the operation of the patient's heart.
In conjunction with such therapy it may be desirable to measure pressure in one or more chambers of the heart. For example, absolute cardiac pressure may be used as an indicator for several potentially lethal cardiac conditions. By measuring cardiac pressure, abnormal conditions may be detected and in some cases the patient's therapy may be modified to compensate for the abnormal conditions. As an example, if cardiac pressure is continuously measured, the operation of an implanted device such as a pacemaker may be adjusted, as necessary, according to conditions diagnosed as a result of the pressure measurements.
Conventionally, pressure sensing devices have been used to measure pressures on the right side of the heart. However, measurements of right side pressure may not provide sufficient indications for detection of conditions such as congestive heart failure, hypertension and mitral valve defects. In particular, left atrial pressure has been identified as an excellent indicator for left ventricular failure.
Obtaining pressure measurements from the left side of the heart presents several challenges. First, access to the left side of the heart must be provided in a safe manner. In addition, the pressure sensors need to be implanted in a manner ensuring that accurate pressure measurements may be made.
Cardiac blood pressure may be measured directly in real time using a pressure sensor device incorporated into a distal end of a permanent or temporary endocardial lead. The lead is typically inserted into the right side of the heart and routed through an opening formed in a septal wall to gain access to the left side of the heart. The lead includes one or more sensors for measuring cardiac pressure on the left side of the heart and, if needed, the right side of the heart. The lead also includes an attachment structure that secures the distal end of the lead to the septal wall.
It will be evident that the diameter of the lead should be as small as possible to minimize trauma to the septum during placement of the lead and to permit the lead to be utilized with other endocardial leads suitable for delivering multi-chamber stimulation, sensing and shock therapy.
As exemplified by U.S. Pat. No. 5,564,434 issued on Oct. 15, 1996, existing pressure sensor devices that are incorporated into endocardial leads use some form of substrate carrying a pressure sensor chip and associated circuitry enclosed within a housing having feedthroughs for making electrical connections. Such pressure sensor devices tend to be bulky and the endocardial leads into which they are incorporated need to have relatively large diameters, for example, greater than 0.104 inch (8 F), compromising their flexibility, making their placement more difficult and time consuming, and limiting the number of leads that may be placed. These sensor devices further tend to be complex, including housing parts that are difficult and expensive to machine. These disadvantages are not offset by any appreciable gain in functionality.
Another drawback of existing endocardial lead pressure sensor devices results from the sealing of the ends of the housing. In an effort to provide a hermetically sealed housing, metal closures are welded to the housing ends. However, the thermal expansion of the dielectric gel or oil used to fill the housing cannot be compensated for by the metal closures, sometimes causing leaks and adversely affecting the sensor's thermal response.